In the production of synthetic resin articles from reactive mixtures, for example, polyurethane articles in which one reactive component may be a di-isocyanate or polyisocyanate and another reactive component can be a di-ol or polyol, use can be made of a so-called mixing head into which the components are pumped and in which the components mix in a mixing chamber which has an outlet or mouth which opens directly into a mold or other cavity in which the shape is imparted to the mixture, or which opens into a further chamber, the latter in turn having a mouth or outlet opening into the mold. The chambers can be swept clean of the mixture by a plunger which can be displaced toward the plane of the mouth and can have an end which lies more or less flush with the mouth in its final position.
The mixing chamber is generally cylindrical and the components of the synthetic resin are generally injected in counterflow into the mixing chamber, i.e. from diametrically opposite orifices. The cleaning plunger which is reversibly displaceable in the mixing chamber, i.e. displaceable back and forth or the control or cleaning plunger which is displaceable in the calming chamber which is downstream of the mixing chamber and usually at an angle thereto, suffices to remove most of the reactive component from the respective chamber but, when the end of the plunger reaches the end of the chamber and lies flush with the mouth, there is the possibility that some droplets of the mixture can form a residue at the mouth of the chamber.
This residue must be removed after each cycle since it rapidly would solidify and be detrimental to the quality of the products made in subsequent cycles or even prevent the reaction mixture from passing effectively into the mold.
It is known, for example, to clean the outlet opening or mouth of a mixing head of the aforedescribed type with wires, blades, rotating brushes, stationary brushes or to use compressed air to remove the residue. The devices provided for this purpose have the drawback that they are largely ineffective or are only incompletely effective and cannot prevent some quantity of the residue from foaming and solidifying and then adhering strongly to the surfaces of the head. Large amounts of foamed residue tend to be produced with earlier systems and the use of earlier apparatus has been found to be labor intensive.